The present invention relates generally to optical systems which generate data signals representative of color images of objects and, more particularly, to a multi-array optical sensor device used in such optical systems which corrects color registration error.
Imaging devices such as color scanners, video cameras, camcorders and the like produce data signals representative of color images of subject objects. Such data signals typically comprise three separate data signals which are representative of red, green and blue color component images of the object. Various methods for generating color component image signals are described in U.S. Pat. No. 4,709,144 of Vincent which is hereby specifically incorporated by reference for all that is disclosed therein.
FIG. 1 is a schematic illustration of a prior art optical scanner device such as the flatbed scanner described in U.S. Pat. No. 4,926,041 of Boyd, et al. which is hereby specifically incorporated by reference for all that it discloses. This optical scanning device 10 images a scan line 12 with an imaging lens assembly 14. The lens assembly 14 images the scan line 12 at an image plane PP. A beam splitter assembly 16 is positioned in the path of the imaging light beam 18 and splits the imaging beam 18 into red, green and blue color component beams 22, 24, 26 having parallel, spaced-apart central longitudinal axes.
The beam splitter assembly 16 comprises a first beam splitter 30 comprising a support member 32, a first transparent plate member 34, and a second transparent plate member 36. The first transparent plate member is coated with a first dichroic coating 38 on one face surface 40 thereof. This coating may be a red light-reflecting dichroic coating. The first transparent plate member is coated with a-second dichroic coating 42, which may be a green light-reflecting dichroic coating, on a second face surface 44 thereof. A first face surface 46 of the second plate 36 is positioned adjacent to dichroic coating 44. The second face surface 48 of plate 36 is coated with a third dichroic coating 49 which may be a blue light-reflecting dichroic coating. The distance X.sub.1 between the first dichroic coating and the second dichroic coating is equal to the distance X.sub.2 between the second dichroic coating and the third dichroic coating.
The beam splitter assembly 16 comprises a second beam splitter 50 comprising a support member 52 a first transparent plate member 54 and a second transparent plate member 56. The second beam splitter may be constructed identical to the first beam splitter with the exception that the order of the dichroic layers is reversed, i.e. red light-reflecting dichroic layer 57 is positioned at the exposed surface of beam splitter 50, green light-reflecting layer 58 is located between the two plates and blue light-reflecting layer 60 is located between plate 54 and member 52. The distances between coating layers 57, 58, 59 and 60 are equal to those on plates 34 and 36. Thus X.sub.1 =X.sub.2 =X.sub.3 =X.sub.4. As described in detail in the above referenced patents, the purpose for using such a compound beam splitter assembly 16 is to equalize the light path lengths of each of the color component beams 22, 24, 26 as they pass through the beam splitter such that the red, green and blue focused images 62, 64, 66 are all focused on a single image plane PP extending perpendicular to the central longitudinal axes of the component beams. However, as graphically illustrated by FIG. 1, the focused images 62, 64 and 66 are not actually located on the same plane due to the chromatic aberration of lens assembly 14. The amount of chromatic aberration depends upon the particular lens design and function. In the illustration of FIG. 1 the red and blue component beams 22, 26 have a nearly identical best focus distance however the green beam 24 has a substantially shorter best focus distance than the other two beams.
An optical sensor device 80 adapted for use in the optical scanning device 10 is shown in plan view in FIG. 2. The optical sensor device 80 comprises a first linear sensor array 82 adapted to sense the red light image 62; a second linear sensor array 84 adapted to sense the green image 64 and a third linear sensor array 86 adapted to sense the blue image 66. The linear sensor arrays 82, 84, 86 are coplaner, equal length and equally spaced apart.
Each linear sensor array is constructed from a single row of adjacently positioned picture elements (pixels) 91, 93, 95 which each generate a data signal indicative of the intensity and duration of light impinged thereon during each operating interval of the optical sensor device 80. Each pixel has an identical size and shape. A typical pixel is square in shape and may be, e.g., 7.000 microns in the longitudinal direction and in the transverse direction. In one typical optical sensor device there are 5,340 pixels in each linear sensor array 82, 84, 86. A typical operating interval for an optical sensor device 80 is 4.6 milliseconds. Thus, at the end of each operating interval, the optical sensor device generates a data signal which includes a first signal portion including discrete data from each individual pixel in the first linear sensor array; a second signal portion including discrete data from each pixel in the second linear sensor array; and a third signal portion including discrete data from each pixel in the third linear sensor array. This data signal is transmitted as by electrical contacts 88, 90, etc. to filtering circuitry, etc. and then ultimately to a personal computer or the like (not shown) which may store the information in the data signal or use the data signal to generate a color display image of the object such as by use of a conventional display monitor (not shown) or a conventional color printer (not shown).
As illustrated in FIG. 1, the optical sensor device 80 is positioned such that the plane of the three linear sensor arrays 82, 84, 86 is coincident with the nominal image plane PP of lens assembly 14. However, due to chromatic aberration of the lens there is not in actuality a single image plane PP because of the fact that the focused color component images 62, 64, 66 do not all lie within a single plane. As previously indicated, color component images 62 and 66 do lie approximately within the same plane but color component image 64 does not lie within this plane. In a typical embodiment using a lens assembly with a magnification ratio of -0.165 image 64 may be spaced a distance of 70-95 microns from the plane of images 62 and 66. In such a situation approximate focus of each component image is obtained by positioning the sensor plane of the sensor device 80 at nominal image plane PP which is approximately halfway between the position of image 64 and images 62 and 66. Accordingly, a slight blurring of each of the images 62, 64, 66 occurs because of the fact that plane PP is not coincident with any of the positions of focused images 62, 64, 66.
Currently, achromatic doublets and other complex lens designs are used to minimize the effect of chromatic aberration. These systems are able to obtain simultaneous focus at two design wavelengths, however, for a continuous spectrum it is still not possible to simultaneously focus three color component beams on the same image plane. Achromatic doublet lenses are also expensive to produce.
Another method used to overcome the focus error problem described above is to design an optical system which utilizes curved reflectors or mirrors rather than lenses. However such a solution is quite expensive and limits the amount of imaging light which is directed onto an optical sensor due to central obscurations in the curved reflectors.
Another method for correcting for lens chromatic aberration is to employ a compound beam splitter having glass plates of different thicknesses to vary the path length of a particular component beam as described in U.S. Pat. No. 5,040,872 of Steinle which is hereby incorporated by reference for all that it discloses. However, a problem with this technique has been that the focused component beams have different magnifications on the image plane leading to color registration error in the resulting data signal.
It would be generally desirable to provide an optical system which overcomes the above discussed problems of color component focus accuracy and which is nevertheless relatively inexpensive to produce, which produces excellent image resolution, and which minimizes the number of lenses needed in the associated lens assembly.